1. Field of the Invention
The present invention relates to a projection lens unit for use in a projector or the like in order to project an image or the like from a light source onto a screen, and more particularly, it relates to a projection lens unit provided with improved means for processing unnecessary light.
2. Description of the Related Art
In recent years, in projectors and the like, in accordance with demand for higher precision of an image such as a high vision image, the projected image is required to have stable quality (picture quality). Hence, a projection lens unit is also required to exhibit high performances.
In such a case, there is a problem of a ghost caused by unnecessary light. That is, of light emitted from a cathode ray tube (hereinafter, simply referred to CRT), unnecessary light incident on the outer periphery of a projection lens causes a crescent ghost to be projected onto a screen.
A projection lens which solves the above problem has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-194065. The projection lens includes a mask provided with an opening having a shape suitable for the shape of an image for shielding unnecessary light. Also, the projection lens includes means for changing the shape of the opening of the mask in accordance with the zooming action thereof. Unfortunately, this approach leads an increased number of components, thereby resulting in a complicated structure and a higher cost.
In view of the above problem, a projection lens unit having a simple structure shown in FIG. 2 and preventing occurrence of a ghost has been proposed.
As shown in FIG. 2, a CRT 1 has a projection lens unit 2 fixed thereto. An image produced by the CRT1 is magnified by the projection lens unit 2 and is then projected onto a screen (not shown).
The projection lens unit 2 is formed by a lens 3, an inner lens barrel 4, and an outer lens barrel 5. The lens 3 is a combination of three lenses consisting of an inside lens 3A, an intermediate lens 3B, and an outside lens 3C, all disposed in and fixed to the inner lens barrel 4.
In order to prevent occurrence of a crescent ghost, the inside lens 3A has a black painted portion 3D disposed along the rim thereof.
The inner lens barrel 4 has a hood 6 disposed on the front thereof, however the hood 6 does not contribute to removing the unnecessary light at all.
In the projection lens unit having the above mentioned structure, the inside lens 3A has the black painted portion 3D disposed along the rim thereof so as to prevent the unnecessary light from being incident thereon, hence a step of a black painting operation is required. That is, an additional working step is needed, thereby leading to a deteriorated working efficiency and an increased manufacturing cost by an amount corresponding to the additional step.